Guide To Electrochemical Technology - Electrosynthesis

Guide To Electrochemical Technology

For Synthesis, Separation, and Pollution Control

72 Ward Road ! Lancaster, NY 14086-9779 Tel: 716-684-0513, 800-657-9176 ! Fax: 716-684-0511

Guide to Electrochemical Technology

for Synthesis, Separation and Pollution Control

Prepared for Electrosynthesis Company, Inc. by Professor Derek Pletcher, University of Southampton

Chemical manufacturers and users are daily faced with decisions associated with the need to improve chemical processes (e.g., increase selectivity, separate difficult mixtures, decrease energy consumption, recover the value of chemicals in waste streams, minimize the discharge of a toxic by-product, etc). Sometimes appropriate technology is available, but often it is necessary to evaluate whether an R&D program is likely to provide an economic and timely solution to the needs of the company. A number of technologies may warrant consideration and possible approaches may include electrolysis. Commonly, however, research directors, plant managers, and other technical support providers have relatively little knowledge of and/or experience with electrochemical technology. This Guide seeks to show that modern electrochemical technology can offer the preferred solution to a range of problems, and several illustrative examples are described. On the other hand, electrochemistry is not the answer for all situations; therefore the Guide also discusses those factors that should be considered to determine whether electrolysis is a viable option.

? Electrosynthesis Company, Inc., 1999. All rights reserved.

72 Ward Road ! Lancaster, NY 14086-9779 ! Tel: 716-684-0513, 800-657-9176; Fax: 716-684-0511

Table of Contents

WHAT IS ELECTROLYSIS? .................................................................................................................. 3 APPLICATIONS OF ELECTROCHEMICAL TECHNOLOGY........................................................ 4 WHY CONSIDER ELECTROLYSIS NOW? ........................................................................................ 6 WILL ELECTROCHEMICAL TECHNOLOGY SOLVE YOUR PROBLEM? ................................. 7 EXAMPLES OF ELECTROLYTIC PROCESSES ................................................................................. 8

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WHAT IS ELECTROLYSIS?

Electrolysis is a unit process in which chemical change results from electron transfer reactions across the electrode/solution interfaces. A voltage applied between the two electrodes in an electrolytic cell drives these reactions. A schematic of an electrolytic cell for the manufacture of Cl2 and NaOH is shown in Figure 1.

e-

cation

Cl2 exchange

membrane

NaOH + H2

e-

Na+

e-

Cl- " ?Cl2

? H2 + OH- # H2O

NaCl

each mole of chlorine formed, two moles of sodium hydroxide must also be produced. Furthermore, it may be seen that the rate of chemical change is proportional to the cell current and the total amount of product may be calculated from the charge passed via Faraday's Law.

Overall, the process objective (the conversion of sodium chloride to chlorine and sodium hydroxide) is achieved through two selective electrode reactions and a selective transport process through the membrane. In common with all processes, a chlor-alkali plant will need several other unit processes to prepare the feedstock for the cell and recover the products. In a chlor-alkali plant, the brine for the cell will be treated to remove hardness and transition ions, and then heated; the products are converted into the appropriate form for sale or downstream use.

ANODE CATHODE

Figure 1. Schematic representation of an electrolysis cell for the manufacture of chlorine and sodium hydroxide.

The feedstock is aqueous NaCl brine, fed to the anolyte compartment between the anode and a cation permeable membrane. Chloride is oxidized to chlorine gas at the anode surface and water is reduced to hydrogen gas and hydroxide ion at the cathode. In order to maintain charge balance, migration of ions must occur, and the membrane is designed to allow the passage of only Na+ from anolyte to catholyte. As a result, sodium hydroxide is formed in the catholyte compartment. Also, in order to preserve charge balance, the rate of transfer of electrons at the two electrodes must be equal ? therefore for

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The technology for the chlor-alkali process, including the design of the cell and the development of highperformance cell components and the other unit processes in a plant, has been described,1 while a further series of books cover the fundamental electrochemistry, electrochemical engineering and membrane chemistry.2,3,4

1 D. Pletcher and F. C. Walsh, Industrial Electrochemistry, Chapman and Hall, London 1990. 2 D. Pletcher, A First Course in Electrode Processes, The Electrochemical Consultancy, Romsey, 1991. 3 F. C. Walsh, A First Course in Electrochemical Engineering, The Electrochemical Consultancy, Romsey, 1993. 4 T. A. Davis, J.D. Genders and D. Pletcher, A First Course in Ion Permeable Membranes, The Electrochemical Consultancy, Romsey, 1997.

APPLICATIONS OF ELECTROCHEMICAL TECHNOLOGY

The present commercial applications of electrochemical technology are many and diverse. Figure 2 is taken from a recent EPRI report, Electrolytic Processes, Present and Future Prospects,5 which reviewed applications under the chapter headings:

! The Chlor-Alkali Industry ! Metal Extraction ! The Manufacture of Inorganics ! The Manufacture of Organics ! The Recycle of Chemicals and

Materials ! Separation and Purification ! Pulp and Paper ! Water and Effluent Treatment ! Atmosphere Control and

Improvement ! Destruction of Toxic Waste ! Soil and Groundwater

Remediation

While all this technology is based on the same fundamental principles, its practical manifestations may be quite different with, for example, cell configurations, electrode materials and sizes, electrolytes and separators, which are each designed to meet the particular demands of the application. Even within a particular heading, the processes may be quite different. For example, consider the manufacture of inorganic chemicals:

! The world production of Cl2 is ~ 45 x 106 tons/year at some 700

5 EPRI Report, Electrolytic Processes, Present and Future Prospects, Report Number TR-107022, December 1997.

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different sites. The chlor-alkali industry is a major user of electric power, consuming ~ 125 x 109 kWh/year, > 1% of the total electricity generated. Not surprisingly, energy consumption is a major consideration in process operation. Also, the scale of the industry warrants the development of highly optimized cell designs and high-performance anodes, cathodes and separators. A modern cell house will be fully automated and will operate continuously without maintenance for several years. ! The world production of potassium permanganate, KMnO4 , is only ~ 20,000 tons/year at a few sites. Energy consumption is a less important consideration in the process economics, and the electrolysis plant is more traditional, without components specifically developed for this process. ! Electrolysis is used for the on-site manufacture of pure arsine, AsH3, for the electronics industry. It avoids storage of highly toxic gas since the cells may be switched on/off as the gas is required. Highly automated and safe units are operated on a very small scale.

A similar story could be told with respect to the manufacture of organic chemicals or with respect to the technology for process stream recycle and environmental protection. Thus, electrolysis is used for the manufacture of the polymer intermediate, adiponitrile, at several sites with a total world production of ~ 300,000 tons/year. It is also used for the production of a number of fine

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